Radiation therapy is a localised treatment designed to treat an identified tissue target (such as a cancerous tumour) and spare the surrounding normal tissue from receiving doses above specified tolerances thereby minimising risk of damage to healthy tissue. Prior to delivery of radiation therapy, an imaging system can be used to provide a three-dimensional image of the target from which the target's size and mass can be estimated and an appropriate treatment plan determined.
Many factors may contribute to differences between the dose distribution determined in the treatment plan and the delivered dose distribution. One such factor is an inconsistency between the patient position at the imaging stage and the patient position in the radiation treatment unit.
Image guided radiation therapy (IGRT) involves the use of an imaging system to view target tissues whilst radiation treatment is being delivered to the target tissue. IGRT incorporates imaging coordinates from the treatment plan to ensure the patient is properly aligned for treatment in the radiation therapy device.
Various medical imaging technologies are used to identify target tissues in radiation therapy planning and IGRT. These include (without limitation); Computed Tomography (CT), Positron Emission Tomography (PET), ultrasound imaging and Magnetic Resonance Imaging (MRI).
The Applicant's prior published international patent application no. WO03/008986 describes a device for use in IGRT which includes the functions of an MRI device in a radiation therapy treatment apparatus and proposes technology for overcoming the problems in doing so. The device comprises a large ring gantry onto which a linear accelerator is mounted and arranged to travel around a target positioned at the isocentre of the ring. An MRI sits in the aperture of the ring gantry sharing the isocentre. A body to be treated is introduced into a treatment space at the isocentre by means of a sliding table.